A large number of catalyst systems that can initiate olefin have been introduced. However, most early work in olefin was done using ill-defined multi-component catalyst systems. It is only in recent years that well-defined single component metal carbene complexes have been prepared and extensively utilized in olefin metathesis.
With the advent of efficient catalyst systems, olefin metathesis has emerged as a powerful tool for the formation of C--C bonds in chemistry. Of importance among the well-defined catalyst systems is the alkoxy imido molybdenum system 1 developed by Schrock and co-workers and the benzylidene ruthenium carbene complexes 2-3 developed by Grubbs and co-workers. ##STR1##
In particular, the ruthenium carbene catalyst systems have drawn a lot of attention, not only because they exhibit high reactivity for a variety of metathesis processes under mild conditions, but also because of their remarkable tolerance of many organic functional groups. However, although these ruthenium carbene catalysts (particularly complexes 2 and 3) have been used in diverse olefin metathesis reactions with remarkable success, further improvements such as better thermal stability, high activity in polar protic solvents, and chiral and cis/trans selectivity, are required to more fully exploit their commercial potential.